For safety purposes, fuel tanks must withstand forces produced by predetermined internal and external pressure differentials, transients and stresses. This is particularly true for tanks made of plastic or high density polyethylene, HDPE. Pressure transients are typically caused by environmental temperature changes. For example, a temperature rise of the tank, or the fuel contained therein, will cause the internal tank pressure to rise and deflection or deformation of the shell of the tank to occur. Uncontrolled deformation and/or expansion of the tank must be avoided to prevent the tank shell from contacting the vehicle body, which could lead to the transmission of noise to the passenger compartment of the vehicle or to damage of the tank shell and ensuing fuel leakage. The weight of the fuel contained within the tank may also lead to a deformation of the shell contour. One method to ensure the shape integrity of the tank is to use retainer straps externally clamping the tank shell. Unfortunately, this causes an increase of the assembly and mounting labor or effort and also increases materials costs, all of which ultimately increases the total production costs. Moreover, such measures provide no or only limited protection against external forces or vacuum or sub-atmospheric pressure conditions inside the tank.
A further known method utilizes one or multiple kiss-off members, or reinforcing structures inside the tank. The structures typically have two opposing indentations projecting inwardly and molded into respective opposing walls of the tank. The indentations “kiss” or engage and are welded to each other at their distal ends thereby decreasing deflection of the shell and increasing the shape stability of the tank. This increases tank rigidity, however, it tends to increase the opportunity of tank wall tears causing fuel tank leaks when internal pressure within the tank is excessive or external forces exerted upon the tank are extreme.
The distal ends of the opposing indentations are engaged by a spot-like or essentially circular weld. Desirably, the engagement area serves not only as a structural feature but also would serve as a yield feature which tears upon excessive forces so that the tank wall or shell does not otherwise tear. The engagement area, however, is difficult to control and/or define in production. Experiments have shown that with this type of point-like spot weld it is very difficult to obtain the desired yield behavior, since the effective wall thickness is larger at the weld than in the surrounding region. Thus, it is observed that often it is the surrounding wall region and not the weld area that yields, resulting in leakage from the tank.